1. Field of the Invention
This invention relates in general to a low pressure inert gas discharge device and to a method of operating same, and more particularly to one in which the luminescence of neon is utilized.
2. Description of the Prior Art
A low pressure inert gas discharge lamp utilizing the luminescence of a positive column has numerous advantages such as less deterioration, longer life, less temperature dependence, and less flux variation after startup, in comparison with a fluorescent lamp.
As neon emits red light, it is suitable as a light source in a facsimile machine or in an optical character reader where a red light source is utilized.
It is well known that there is flickering, commonly termed moving striations, in the positive column of a low pressure inert gas. Such striations depend upon the value of the discharge current; there are upper and lower limits for the discharge current which cause the striations to occur. Consequently, it is required that the value of the discharge current be below the lower limit or above the upper one in order to obtain a stabilized discharge with no striations.
Usually a discharge current whose value is below the lower limit will not produce sufficient light output because of its small value and is thus of no practical use, whereby it is required that the value of the discharge current be above the upper limit.
This upper limit is established by the following formula, called Pupp's critical current: EQU Ic=c/p,
wherein:
Ic=critical current, PA1 c=constant value peculiar to a given inert gas, and PA1 p=gas pressure (Torr). PA1 .gamma.=an additional constant value peculiar to a given inert gas.
The above formula has been further developed by Rutscher and Wojaczek, as follows: EQU Ic=c/p.gamma.,
wherein:
For neon, c=7 and .gamma.=1.
These formulae have been derived from direct current discharge, and are therefore not applicable to alternating current discharge because the current value so determined may be above the upper limit at a certain moment and less than such limit at another moment.
It is thus difficult to determine the upper and lower limits for critical currents in an alternating current discharge mode. With respect to a high frequency discharge, however, as the alternating speed of the electrical polarity of a discharge current is higher than the speed of ambipolar diffusion, the ion density does not vary in accordance with the alternation of the polarity of the discharge current; in other words, the ion density is almost constant. Therefore, critical lower and upper current limits can be established.
The value of the critical current depends upon the gas pressure, which is determined in consideration of luminous efficiency and life, while it is required that the value of the discharge current be more than that of the upper critical current limit.
The design of a lamp, a lighting apparatus, or a range where a lamp is applicable is limited by the critical current. It is thus desirable to reduce the value of the critical current in order to minimize this limitation.
Among low pressure gas discharge lamps where the luminescence of an inert gas is utilized, gaseous impurities which have an undesirable effect on emitting light, starting, and lighting are minimized using getters. The impure gas contained in such a lamp would cause the lamp to start with difficulty. If the impure gas contains an atom or a molecule whose excitation potential is lower than that of that of the inert gas, the energy supplied to the lamp is first consumed by such an atom or molecule. Light which is unnecessary or undesirable is then emitted, and subsequently the lamp becomes poor in both its colorimetric purity and its efficiency. For example, an energy of about 19 (ev) is needed for a low pressure neon discharge lamp to emit red light at a wave length of 640 (nm). If a molecule of nitrogen (the resonance excitation potential for N2 is 1.6 (ev) and that for N is 10.2 (ev)), of oxygen (the resonance excitation potential for 0 is 9.1 (ev)) or of hydrogen (a resonance excitation for H is 12.2 (ev)) is contained in the lamp as an impure gas, an energy of about 13 (ev) is sufficient for such an impure gas to emit light. Consequently, the light emitted from such an impure gas and that emitted from the neon gas mix with each other. Under these circumstances, a red light emitting neon lamp which has both excellent colorimetric purity and a high efficiency cannot be obtained. Additionally, an impure gas which is produced in correspondence to the consumption of the cathode material causes the discharge to be unstable and reduces the life of the lamp.